AMON, archives, etc

Question about configuring NOD32. Under the "Setup" tab in the "Scan" section.. you can pick what to scan. Files, boot sectors, archives, email files, etc. Default is files, boot sectors, and system memory. Why would I want to scan archives or runtime packers or self-extracting archives? My thought is that once I actually UNPACKED one of those files, then AMON would catch the virus.. wouldn't it? Or is the idea to not be sending people archives that may have viruses in them?

Also, is there some way to configure NOD32 so that manual scans only scan files, memory, etc... but AMON will scan files, archives, runtime packers, etc.? That way.. full system scans will be fast, and though it might miss a virus inside an archive, if I were to copy or move that archive, AMON would pick up the virus inside. I actually would prefer to never run a full system scan. I'd like to just run it once to make sure my system is clean, then let AMON/EMON/IMON/DMON/etc automatically find viruses. I guess my question is.. once you've done a full system scan and you're clean, why do a full system scan ever again, as long as the automatic *MON engines are running??